Petroleum-based resins such as polyethylene terephthalate, nylon, polyolefin, and soft polyvinyl chloride (PVC) are still widely used as materials for various applications. However, since such petroleum-based resins are not biodegradable, they may cause environmental contamination including the emission of a large amount of carbon dioxide, which is a global warming gas, when disposed. In addition, due to the gradual depletion of petroleum resources, recently, the use of a biomass-based resin, particularly, a polylactide resin is extensively examined.
However, since such a polylactide resin has insufficient heat resistance, humidity resistance, or mechanical properties when compared with a petroleum-based resin, there was a limitation on the application field or use thereof.
In 1987, Ikeda et al. found that a mixture of the enantiomers of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) produces a stereo-complex crystal having a melting temperature (Tm) of 230° C., i.e., which is higher by 50° C. than PLLA and PDLA (see Ikeda, Y et al., Macromolecules, 1987, 20, pp. 906-908).
The formation of the stereo-complex is considered due to van der Waals interaction between enantiomeric polymer chains. The crystallinity of the stereo-complex depends on the composition and molecular weight of PLLA and PDLA. With the PLLA and PDLA polymers having low molecular weights, high crystallinity of the stereo-complex may be obtained. On the contrary, with a polymer having a high molecular weight, it is observed that both homo-chiral and stereo-complex crystals are formed at the same time.
As the synthesis of a stereoblock polylactide (sb-PLA) by the two-step polymerization of lactide, as shown in the following Reaction 1, a method including a first step of obtaining a PDLA prepolymer by a ring opening polymerization of D-lactide using an alcohol as an initiator, and a second step of obtaining a sb-PLA by polymerizing L-lactide using the PDLA prepolymer as an initiator, may be illustrated.

The conventional method as above Reaction 1 includes processes of isolating the PDLA prepolymer during the first step, removing remaining monomers included therein, and performing the polymerization of the second step (prepolymer isolation method), such reaction processes are complicated to cause long preparation time and the increase of production cost is inevitable. If the removing process of the remaining monomer can be omitted, a sb-PLA may be synthesized by an even simpler method, and cost reduction may be attained.
In addition, during the latter part of the polymerization of the first and second steps, if monomer conversion is increased to the extent that a monomer concentration is close to an equilibrium monomer concentration, a chain transfer polymerization as shown in the following Reaction 2 may compete. As a result, chain scrambling may occur, and the multi blocking and oligomerization of polymer chains may be promoted. In particular, since the formation of the oligomer may become a factor bringing about the decrease of a molecular weight during melt molding process of a produced polymer, this procedure should be avoided.

The present inventors have endeavored to examine the two-step polymerization method which is a strong candidate for replacing the prepolymer isolation method, and found that a sb-PLA may be very easily synthesized by applying a novel method using a one-pot synthesis. In addition, it was found that the multi blocking and oligomerization of polymer chains due to chin scrambling may be prevented by decreasing chain transfer during polymerization by adding stepwise the polymerization monomer at the second step while controlling monomer conversion during the one-pot synthesis.